FIG. 1 (Prior Art) is a simplified cross-sectional diagram of an old power bipolar transistor structure 1 that was used in the early days of bipolar transistors. The backside of the die is covered with a solderable metal. In the illustrated example, the backside surface of a wafer of devices is covered with a layer 2 of aluminum or titanium. A layer 3 of nickel is then formed over the aluminum or titanium to act as a barrier. A layer 4 of silver or gold is then formed by an evaporative deposition process onto the layer of nickel. Thereafter the wafer is diced so that each die has the layers of backside metal as illustrated in FIG. 1. The silver or gold layer 4 is then bonded to a substrate 5 of a package using an amount of soft solder 6. The backside metal structure forms the collector electrode of the bipolar device. An emitter electrode 7 and a base electrode 8 are disposed on the topside of the die. Each electrode involves an aluminum layer 9, which is covered by a titanium layer 10, which in turn is covered by a barrier layer of nickel 11, which in turn is covered by a silver layer 12. These layers are patterned and etched to form the emitter and base electrodes.
FIG. 2 (Prior Art) illustrates the topside emitter electrode 7 and the topside base electrode 8. A first post 15 and associated clip 16 provide an electrical contact and connection from the emitter electrode 7 to a conductor in the package. A second post 17 and associated clip 18 provide an electrical contact and connection from the base electrode 8 to another conductor in the package. The clips are attached to the silver on the top of die using amounts of soft solder 18.
With the advent of power field effect transistors, bond wires and wire bonding was used to make electrical connections between the gate and source pads on the die and associated conductors in the package. Clips were typically not used because the geometries of the pads on the die were too tight, and a clip may have shorted the pads together and caused other problems and failures. Currents were typically less than 50 amperes through a pad and were not as great as in the part power devices that were packaged in the manner illustrated in FIG. 1. A prior conventional thyristor, for example, may have been required to conduct 500 amperes through an electrode. Accordingly, due to the lower current requirements of the early field effect transistors, bond wires were adequate. Over the years since the advent of power field effect transistors, however, currents have steadily increased. To accommodate the increased currents, more and more bond wires have been provided to handle more and more current.
FIG. 3 (Prior Art) is a very simplified illustration intended to show that a great many bond wires 19 may be provided between the source electrode pad 20 of a power field effect transistor device 21 and its package. Fewer bond wires 22 may be provided for the lower current gate electrode pad 23, but there still may be multiple bond wires provided for this connection. It is now often common to have twenty or thirty bond wires attached to the source electrode pad in some high current power field effect transistor devices. Having this many bond wires on a single pad may introduce crowding problems and other assembly problems. The use of bond wires is nonetheless desirable due to the flexibility of how the connections can be made.